Handout(ALBUQUERQUE, N.M.) -- Relatives of a New Mexico teenager accused of killing his mother, father and younger siblings with an assault rifle, then telling police he hoped to shoot up a Walmart, are described as stunned by his actions and "heartbroken over this senseless tragedy."

Though surviving relatives conceded in an unsigned "family statement" that 15-year-old Nehemiah Griego is a "troubled young man," the statement also described him as an outgoing boy who loved music and hoped one day to serve in the military.

"We know him as a bright, curious and incredibly talented young man. He was a brother, nephew, grandson and cousin," said the statement, which was obtained by ABC News affiliate KOAT in Albuquerque from former New Mexico state Sen. Eric Griego, the suspect's uncle.

"We are deeply concerned about the portrayal in some media of Nehemiah as some kind of a monster," said the statement. "It is clear to those of us who know and love him that something went terribly wrong. Whether it was a mental breakdown or some deeper undiagnosed psychological issue, we can't be sure yet. What we do know is that none of us, even in our wildest nightmare, could have imagined that he could do something like this."

Nehemiah Griego, the son of an Albuquerque pastor, had plans to kill his family, his 12-year-old girlfriend's family and local Walmart shoppers for weeks before he acted on the impulse on Sunday, according to police.

"Nehemiah said after killing five of his family members he reloaded the weapons so that he could drive to a populated area to murder more people," read a police report from the incident released on Tuesday.

"Nehemiah stated he wanted to shoot people at random and eventually be killed while exchanging gunfire with law enforcement," the report said.

However, after allegedly killing his family members, Nehemiah ended up spending most of his day with his girlfriend rather than going to the Walmart, Bernalillo County Sheriff Dan Houston said on Tuesday.

Nehemiah later was arrested and is expected to face adult charges of murder and child abuse resulting in death. He waived his right to arraignment in adult court Tuesday and a judge ordered him held without bond.

The district attorney's office and Nehemiah's public defender now are preparing to face a grand jury, KOAT reported.

"We never had a case like this, as far as I know, in the state of New Mexico," District Attorney Kari Brandenburg said, "so I can't compare this to any other case."

Police also are considering charging Nehemiah's girlfriend, who they have not named publicly.